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name: custom-dimensions description: "How to find and query property-specific custom dimensions in GA4."

Custom Dimensions and Event Parameters

How to find and query property-specific custom dimensions in GA4.

The core rule

Custom dimensions are property-specific. Never guess their names. Always call search_schema("custom") first — it will show every registered custom dimension for this exact property.

Two valid syntaxes

GA4 supports two ways to access custom data:

Registered custom dimensions — appear in the schema by their registered name. Find them with search_schema. Use the name exactly as returned.

Unregistered event parameters — accessed inline with:

• customEvent:parameter_name for event-scoped parameters
• customUser:property_name for user-scoped properties

The parameter_name must exactly match what your tracking code sends. If the property has not collected this parameter, the API returns an error.

Why guessing fails

The model cannot know what custom parameters a GA4 property uses — they are defined by the site's own tracking code and GA4 Admin settings. customEvent:page_category works on one property and fails on another depending on whether that property's tracking sends a page_category parameter.

Workflow

1. Call search_schema("custom") — lists all registered custom dimensions.
2. If the one you need appears, use its exact API name.
3. If it does not appear, ask the user what event parameter name their tracking code uses, then query with customEvent:that_name.
4. If the query still fails, the parameter is not collected by this property.

Scope matters

Scope	Syntax	Use for
Event	customEvent:name	per-hit data (page category, product id, etc.)
User	customUser:name	user attributes (plan, cohort, customer type)
Session	registered name only	session-level custom groupings

Mixing scopes can cause incompatibility errors — see the compatible-combinations skill.